1. Field of the Invention
This invention relates to a processing solution for a metal image-forming material comprising a support, a thin metallic layer composed mainly of aluminum on the support, and a photosensitive resin (or a photoresist) layer on the thin metallic layer, and more specifically, to a developing solution used to develop the metal image-forming material and to a reducer solution used to reduce halftone dots (i.e., to reduce the size of the dot image) after development.
2. Description of the Prior Art
Japanese patent application (OPI) No. 139720/75 (corresponding to U.S. patent application Ser. No. 571,817, filed Apr. 25, 1975, now U.S. Pat. No. 4,008,084) discloses a metal image-forming material comprising a support, a thin metallic layer composed mainly of aluminum on the support, and a layer of a photosensitive resin on the metallic layer. When this metal image-forming material is exposed through an original image using actinic radiation and then brought into contact with an alkaline solution, the resin layer is dissolved or swollen at the exposed or unexposed portion depending on the property of the photosensitive resin layer. As a result, the alkaline solution contacts the metallic layer which is then dissolved to form the corresponding image. The pattern so obtained has high contrast and superior resolving power.
When this metal image-forming material is exposed through a halftone original and developed, substantially the same image as the original or an image whose light and dark areas have been substantially completely reversed from the original is obtained. This image can be used as an original to be printed on a printing plate. When it is desired to change the size of the halftone dots partially so as to obtain a better print, an operation termed "halftone dot reduction" is performed. Halftone dot reduction is well known and described in, for example, B. R. Halpern, Color Correction for Offset Lithography, section 20, pp. 32-35, (1956) and E. Jaffe, Halftone Photography for Offset Lithography, third Ed., Chapter XIII, pp. 140-147, (1964), both published by Graphic Arts Technical Foundation, Inc., New York. In dot reduction of a lithographic material containing silver halide which has heretofore been used to prepare a halftone original, the reducer solution acts on all of the surfaces of silver grains which form the image, and therefore, the optical density of each of the halftone dots as a whole decreases. In contrast, since in a metal image-forming material the resin layer is present on top of the metal layer which forms the images, the selection of a reducer solution which substantially cannot permeate through the resin layer makes it possible to cause the reducer solution to act on the halftone dots only at surfaces of such where the dots neither contact the photosensitive resin nor the support, and thus to change only the size of the area of the halftone dots.
However, when the developer solution disclosed in Japanese patent application (OPI) No. 139720/75 or the reducer solution used for the above purpose is merely an alkaline solution, vigorous foaming at the time of dissolving (etching) the metal occurs, and the resulting image is impaired. The foam generated adheres to the surface of the metal layer, and supply of the alkaline solution to these parts is hampered. This retards the etching of the metal. If the etching is prolonged until these parts are completely removed, those parts to which the alkaline solution has been supplied in a normal manner become excessively etched.
On the other hand, Japanese patent application (OPI) No. 2925/75 (corresponding to U.S. patent application Ser. No. 350,372, filed Apr. 12, 1973) discloses an etching solution obtained by adding sodium perchlorate to an alkaline solution, which is used for an image-forming material composed mainly of tellurium. As is well known, however, perchloric acid, chlorous acid, chloric acid, and the salts of these acids tend to undergo an explosive decomposition, and special care is required in their storage and handling. Some chlorate and perchlorate salts are designated even as first-grade dangerous articles under the Japanse Law of Fire Defense.